In many fields of environmental monitoring, workplace monitoring, pollution control, and occupational health monitoring, it is necessary to continuously measure the aerosol content of a local environment. Where real-time high-sensitivity measurements are desired, instruments are used that are capable of measuring the sizes of individual particles at high rates (typically thousands of particles per second) so that an accurate assessment of both the concentration of particles and their size distribution can be achieved.
The instruments that are commonly used for this purpose are ‘optical particle counters’ (OPC). These use an intense light source (usually a laser) to illuminate a narrow column of sample air drawn through a sensing chamber by an electrical air-pump. The air column is sufficiently narrow that the volume of air illuminated by the laser beam—often referred to as the particle ‘sensing zone’—rarely contains more than a single particle, and the pulse of light scattered by this particle as it crosses the laser beam is recorded as a particle count, the magnitude of the pulse being indicative of the particle size according to a calibration function. The calibration function depends on factors such as the laser wavelength and beam power and the solid angular range over which the scattered light from the particle is recorded.
There are many excellent commercial varieties of OPC manufactured by companies such as Met One Instruments (USA), Grimm Aerosol Technik GmbH (Germany), and Casella Measurement Ltd., (UK). In all these instruments, the air-pump provides the necessary suction to draw the ambient air through a narrow sample tube and deliver the resulting column of air through the laser beam. The pump itself must be powerful enough to generate the required pressure drop inside the sensing chamber to draw the external air in through the sample tube. Furthermore, since the pumps are normally required to be protected from particulate contamination in the air, they are generally preceded in the air-flow by a high efficiency particle filter and this too will present a flow impedance which the pump must overcome. These features of conventional OPC's result in several drawbacks; firstly, the pumps used must overcome the pressure drops in the sample tube and particle filter, and this has implications for pump size and current consumption (and, by extension, for battery-life in standalone of hand-held units); secondly, the particle filters need to be periodically replaced (incurring further cost to user); and thirdly, the air sample handling system comprising sample inlet tube, vent tube, interconnecting tubing and filter, etc.), can be relatively complex to manufacture and assemble, with implications for final unit cost.
WO2008140816 (UNGER) describes a compact low cost particle sensor wherein the flow of fluid is directed to a detection zone via a series of baffles. The detection zone is basically a detector located directly under the laser beam which collects radiation scattered by particles passing through the beam. A problem with this device is that the detector is not able to distinguish the number or size of particles passing through the, rather large, detection zone. This device is only able to detect that at least one particle has passed through the detection zone, and in this regard it is little more than a portable smoke detector. This device cannot be used to accurately count the number of particles passing through the detection zone
There is a need for an apparatus and method of aerosol monitoring, having single particle sensitivity, which requires no air-pump or particle filter, and which is mechanically very simple, thus offering the prospect of manufacture using a small number of parts.
GB 0917444.2 (University of Hertfordshire) describes one type of apparatus for the detection of a fluid-borne particle in an optically defined sensing zone wherein a first reflector or refractor means is adapted to direct radiation scattered from the fluid borne particle passing through the beam of radiation in the optically defined sensing zone into the first detector means and wherein the second reflector or refractor means is adapted to direct radiation scattered from the fluid borne particle passing through the beam of radiation in the optically defined sensing zone in to the second detector means.